The present invention relates to a new and improved construction of a smoke detector having a radiation source which transmits radiation in a radiation region encompassing an expanded or extended solid angle region. At least one radiation receiver is arranged externally of the direct radiation region of the radiation source and receives radiation which is scattered at smoke particles in the radiation region. An evaluation circuit is connected with the radiation receiver for signal transmission when the radiation received by the radiation receiver has exceeded a predetermined value.
With such type smoke detectors, as the same have been employed for instance in the fire detection art, the radiation, depending upon the nature of the smoke particles to be detected, can be selected so as to be within the visible, infrared or ultraviolet wavelength region. The solid angle region of the transmitted radiation and the arrangement of the radiation receiver are advantageously selected such that there is realized as good as possible efficiency, i.e., the receiver already is capable of taking-up as much as possible of the scattered radiation even with low smoke density. A suitable construction of smoke detector for this purpose has been disclosed, for instance, in Swiss Pat. No. 592,932 and the aforementioned related, commonly assigned U.S. application Ser. No. 777,397, now U.S. Pat. No. 4,181,439. Evaluation circuitry suitable for such smoke detector and that of the present invention have been disclosed in the commonly assigned U.S. Pat. No. 3,760,395, granted Sept. 18, 1973 and Swiss Pat. No. 520,990. The disclosure of the foregoing patents and applications is incorporated herein by reference.
Such smoke detectors are, however, associated with the drawback that their sensitivity decreases with increasing temperature. This phenomenon is primarily predicated upon characteristic changes of the employed components in the presence of a temperature increase, especially due to the reduction in the sensitivity of standard radiation receivers above a certain permissible maximum temperature and owing to the reduction of the radiation output of conventional semiconductor-light sources with increasing temperature. To highlight this, there has been shown in accompanying FIG. 2, based upon the curve A, this reduction of the smoke sensitivity of a prior art smoke detector as a function of the temperature T. The ordinate d represents the smoke concentration at which such smoke detector delivers a signal. It will be seen that the smoke density d needed for signal transmission already rises slightly above 30.degree. C., i.e., the smoke detector becomes less sensitive.
In practice this leads to the result that in the case of a fire accompanied by a rapid temperature increase such smoke detector first responds too late or not at all. In order to nonetheless ensure that there will be transmitted an alarm signal, it is known, in the smoke detector art, from U.S. Pat. Nos. 3,226,703 and 3,555,532 to provide an additional temperature switch or contact.
It is also already known to use a bimetallic strip in a smoke detector for signal transmission upon exceeding a predetermined maximum temperature. Such an arrangement has been disclosed in U.S. Pat. No. 3,430,220, granted Feb. 25, 1969 to Albert Deuth. The scattered light smoke detector therein disclosed has the light emitted by a lamp screened by means of a diaphragm to such an extent that there is formed a narrowly confined, almost point-like radiation region from which a photocell receives the radiation scattered by the smoke particles. Within the detector housing there is provided the bimetallic strip which slowly bends in the presence of a temperature increase. Upon reaching a predetermined threshold temperature this bimetallic strip moves quite suddenly into the radiation region of the lamp, so that the light which is reflected and scattered at the surface of the bimetallic element tends to superimpose upon the light that is scattered by the smoke and impinges at the photocell. Since this sudden moving or rocking of the bimetallic element into the radiation region occurs at a certain critical temperature, the irradiation of the photocell or photoelement likewise is suddenly increased and at this critical temperature there is triggered an alarm. However, what is disadvantageous with this prior art construction is that prior to reaching this critical temperature the sensitivity of this described smoke detector decreases, which, in turn means that with increasing temperature, prior to reaching the critical threshold, there is initially required an increasingly greater smoke density for triggering an alarm. This sensitivity decrease with increasing temperature is, however, totally undesired in practice, since upon occurrence of a fire, practical experience has shown that the development of smoke also usually is accompanied by a temperature increase. Such state-of-the-art fire alarms are afflicted with the drawback that when encountering this situation, as frequently occurs in practice, they first trigger an alarm signal too late in time. The slow sensitivity decrease of such smoke detectors, which already begins slightly above the room temperature, in other words already considerably below the critical maximum temperature of say around 70.degree. C. for instance, is in no way overcome.
It is also known to the art, for instance from U.S. Pat. No. 3,469,250 and British Pat. Nos. 1,485,790 and 1,486,535, to construct the electrical circuit, by using temperature-sensitive elements, for instance thermistors, such that the threshold value of the output signal of the radiation receiver, at which there is triggered a signal, is reduced with increasing temperature. Such evaluation circuit requires, however, an increased number of components and is therefore correspondingly costly and prone to disturbances.